Dear Santhosh,
This is not available OOTB, but you can implement it in different ways.
A simple example is that, you publish different API for different merchant but use fragment to reuse the API code, then the metrics of API will be the metrics of the merchant.
Alternatively, you may do something similar as App in the API developer portal, define different Apps (for different merchant) to include the same API, call the api with parameter apikey to identify different App(merchant). You would need to associate the apikey with metrics (apikey is just a concept in portal, you can rename it as merchantID, or whatever)
Regards,
Mark